1. Field of the Invention
This invention relates to a drying etching method and more particularly, to an improvement in the drying etching of semiconductor and other materials.
2. Description of The Prior Art
As is known in the art, the sizes or widths of transistors and wirings play an important role in the fabrication of high density semiconductor integrated circuits. By the reduction in size, a fine pattern with a size of not larger than 1 .mu.m has now been realized. The realization of such a fine pattern depends greatly on the drastic developments of two techniques including a lithographic technique and a dry etching technique.
The drying etching technique makes use of a phenomenon where an appropriate gas is applied with a high frequency of 13.56 MHz from a RF power supply to generate a reactive plasma or radicals with which a material to be etched is etched. For the formation of a fine pattern, it is ordinary to employ a photoresist pattern as a masking material. In recent years, the RIE (reactive ion etching) technique has been predominantly used wherein a self-bias voltage (V.sub.do) is utilized to take up a plasma from the reactive ions, thereby carrying out anisotropic etching. The reason why the frequency of the RF power supply used is 13.56 MHz is that such a frequency is one assigned to by the radio law and thus, little problem is involved if the radio frequency may be leaked, so that a simple shielding device is usable.
FIG. 10 is a schematic view of a known dry etching apparatus realizing the RIE technique. The apparatus generally indicated by A includes a metallic chamber 1 which has a feed port 2 through which a reactive gas is charged. The chamber 1 also has a discharge port 3 through which the gas is discharged. By controlling the amounts of the gas being charged and discharged, the chamber is controlled at an appropriate pressure, for example, of several hundreds mTorr. The chamber 1 has an anode 4 and a cathode 5 as shown. On the cathode 5 is mounted a material 6 to be etched which has a resist pattern (not shown) thereon. The cathode 5 is connected through a blocking capacitor 7 to a RF power supply 8 to supply electric power to the gas.
In operation, when a potential from the RF power supply 8 is applied to the reactive gas in the chamber 1, glow discharge takes place between the anode 4 and the cathode 5 as is schematically shown as in (a) of FIG. 11 to produce electrons and ions thereby generating a plasma. Since the sample is mounted on the cathode, the electrode area of the anode contacting the glow discharge becomes greater than that of the cathode. At the same time, the mobility of the electrons in the plasma is much far greater than that of the ions, so that the electrons are flown into the cathode, thereby causing the blocking capacitor to be negatively charged. Thus, the cathode is negatively biased. This bias potential is called self-bias voltage, V.sub.do. The distribution of the potential in the plasma in this condition is schematically shown as (b) of FIG. 11. As shown in (b) of FIG. 11, the plasma is divided into a bulk region where the potential is constant and sheath regions where the potential is abruptly changed in the vicinity of the electrode owing to the self-biasing. The ions are produced mainly in the bulk region. The ions produced in the bulk region is entered from a bulk/sheath boundary into the sheath region and is accelerated with the negative potential caused by the self-biasing in the sheath region to cause the sample material to be hit and etched. This etching is anisotropic in nature and is called anisotropic etching.
The manner of producing the sheath region as a result of the generation of the negative self-bias voltage at the cathode is described in more detail.
As stated above, the mobility of the electrons generated by the glow discharge is greater than that of the ions. In general, the current-voltage characteristic in this condition is similar to that of a rectifier of the type where a leakage current is great. When the potential from the RF power supply is applied to the cathode, the electrons having a greater mobility are passed in great numbers toward the cathode with the positive potential at the positive half cycle of the frequency. The ions with a smaller mobility are flown only in small numbers toward the cathode whose potential is rendered negative at the next half cycle. Thus, the electrons and the ions do not reach the equilibrium in number. This results in the negative charge of the blocking capacitor, so that spatial charges of the electrons are produced in the cathode and a negative potential is produced in the cathode until the electrons are repulsed, with a reduction in number of excess electrons. In this manner, after several cycles, the number of electrons entering the cathode becomes equal to that of the ions entering the cathode. A negative bias potential is produced at the cathode so that the net current becomes zero on the time average, thus leading to a stationary state. This potential is called self-bias. Owing to the existence of the self-bias, the region where little electrons exist with the existence of the ions alone is formed in the vicinity of the cathode. This region is called the sheath region in which the potential is abruptly changed.
On the other hand, since the electrons tend to be diffused outside in the plasma, they are apt to be deficient in number. Hence, the potential becomes slightly positive. This potential is called plasma potential (V.sub.p). The change of the cathode potential in this state in relation to the variation in time is shown in FIG. 12. FIG. 12 demonstrates that during the positive cycle wherein the potential is greater than the plasma potential, +V.sub.p, the electrons are passed into the cathode and during a negative cycle where the potential is smaller, the ions are moved. However, a time at which only the ions are moved is very long and the etching reaction takes place by the action of the ions passed into the cathode.
Goto et al proposed a novel dry etching apparatus as is particularly shown in FIG. 13 (SSDM, 1990, pp. 1147-1150). In the figure, like reference numerals indicate like members or parts as in FIG. 10. In this apparatus, RF power supplies 8 and 12 are connected not only to the cathode, but also to the anode, respectively. The frequency from the RF power supply at the cathode side is controlled at 10 to 50 MHz and the frequency from the RF power supply at the anode side is as high as 150 to 200 MHz. The RF power supply at the anode side is used for the generation of plasma, like ECR and MERIE, by which power of about 1 kW is supplied to produce a plasma with a high degree of ionization. On the other hand, the RF power supply provided at the side of the cathode is to attract the ions. More particularly, the RF power supply serves to attract the ions necessary for etching from the plasma produced by means of the RF power supply at the side of the anode.
An increasing degree of integration of semiconductor is now requiring fine processing techniques wherein semiconductors are processed on the order of submicrons. In this connection, however, known etching techniques involve the problem that the reduction in size of transistors invariably entails a thin gate oxide film of transistors, so that the gate oxide film is liable to undergo dielectric breakdown during the course of drying etching. The reason for this is considered as follows: when the RIE apparatus is used for etching, anisotropic etching is effected by means of the accelerated ions and charge-up takes place during the etching; and a voltage applied to the gate oxide film is made greater, exerting a stress on the film. Especially, gate oxide films of chips around a wafer is liable to undergo the breakdown. In addition, with known etching techniques, bombardment of high energy ions against a silicon substrate will cut the bonds of the substrate, thereby degrading the element. Moreover, if Cl.sub.2 gas is used, for example, for the dry etching of a gate electrode, the resultant radical component should not substantially take part in the etching of SiO.sub.2 film. Nevertheless, a selection ratio of a polysilicon gate and a SiO.sub.2 underlayer is, at most, about 10:1 owing to the presence of the high energy ions. Thus, this technique is not suitable for etching of the gate polysilicon.
In order to reduce damages of materials to be etched due to the charge-up of the ions so that dielectric breakdown of the gate oxide film is suppressed from occurring, it is sufficient to prevent generation of the high energy ions. For the suppression of the high energy ions, it is effective to lower the RF power. However, the lowering of the RF power results in the lowering in degree of ionization of the plasma, thereby causing the radicals to be reduced in number. This eventually leads to a lowering of the etching rate with a poor efficiency. In the Goto et al apparatus, this is solved by the provision of a RF power supply at the side of the anode. This apparatus is so designed that while the power of the RF power supply at the side of the anode is increased to about 1 KW to maintain plasma-generating conditions, the radicals are not reduced when the RF power at the side of the cathode is decreased. As will be appreciated from the above, in the Goto et al apparatus, a semiconductor substrate is etched at a low energy by taking up the ions from the plasma, which is produced by means of the high power supply provided at the side of the anode, by the use of a sheath electric field produced from the low RF power supply at the side of the anode.
However, the Goto et al apparatus has the following problem to solve. In general, the arrival energy distribution of the ions to a substrate electrode is so wide as a saddle-shaped distribution, as shown in FIG. 14a, which has a central point of the energy obtained by acceleration of the ions by self-biasing. In the Goto et al apparatus, the power from the RF power supply at the side of the cathode is lowered to decrease the self-bias so that the ion energy distribution is, as a whole, shifted toward a lower energy side as is shown in FIG. 14b. In this arrangement, if a gate electrode is etched, for example, the ion energy dependence of the etching rate between the polysilicon to be etched and the SiO.sub.2 underlayer is as shown in FIG. 15. From FIG. 15, it will be seen that when the ion energy is in the range of about 50 to 60 eV as shaded in the figure, the selection ratio is high and the etching rate of the polysilicon is great, so that the etching can be efficiently carried out. However, when the ion energy is higher than 60 eV, the etching rate of SiO.sub.2 becomes great with a poor selection ratio. On the contrary, when the ion energy is lower than 50 eV, the etching rate of the polysilicon becomes so small that the etching does not proceed efficiently and the anisotropy becomes poor. In the energy distribution of FIG. 14b obtained by the Goto et al apparatus, the average arrival energy is small enough to suppress damages of the substrate. However, since the width, .DELTA.E, of the energy distribution is not so small. Accordingly, the apparatus is not so improved in the selectivity, anisotropy and etching efficiency. In addition, the Goto et al apparatus is disadvantageous in that it requires two power supplies and two matching boxes with a difficulty in matching.
The frequency from the RF power supply is considered to be other factor for producing high energy ions. If the frequency of the RF power supply is so low as 13.56 MHz as has been currently employed, the self-bias becomes large and the width of the arrival energy distribution of the ions to substrate also becomes large, eventually leading to the generation of the high energy ions. In contrast, when the frequency of the RF power supply is made high, the self-bias is made small simultaneously with a narrow distribution of the ion energy as is shown in FIG. 14c. Thus, it becomes possible to suppress the generation of the high energy ions. This entails not only a reduced damage of the substrate, but also realization of dry etching with high selectivity, anisotropy and etching efficiency.